There has been known a communication system performing wireless communication by using a beacon signal between a master unit and a slave unit (for example, the beacon wireless communication system defined in IEEE802.15.4). The beacon signal is a signal indicating the timing of transmission/reception of a signal, and includes an identification number of the slave unit to which the data is transmitted. In the wireless communication of above standard, schedule information for managing an access time is not included in the signal transmitted from the master unit to the slave unit. Thus, the slave unit which receives a beacon signal calculates the time to access the master unit based on its own identification number included in the beacon signal and the like. The signal period of the beacon signal is divided into the interval for signal transmission from the master unit to the slave unit, the interval for signal transmission from the slave unit to the master unit, and the non-communication interval which is not used for communication.
The data signal for performing notification to the user via the slave unit is included in the signal transmitted from the master unit to the slave unit for example. The signal transmitted from the slave unit to the master unit includes a participation request to the communication with the master unit, a withdrawal request from the communication with the master unit, and a handover request among a number of master units, for example. Here, when requests of the signal transmission from a number of slave units to one master unit are issued at the same time, the access is performed by avoiding the collision by the CSMA-CA (Carrier Sense Multiple Access with Collision Avoidance) system. In the CSMA-CA system, if one slave unit is emitting the radio wave when another slave unit sends a signal, the collision between slave units is avoided by accessing the master unit after the communication is queued for a certain period which is calculated randomly.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2009-49924